Nonlinear signal transmission system



De- 25, 1951 l A. J. TORRE ET Al. 2,580,052

NONLINEAR SIGNAL. `TRfxNSn/IISSION SYSTEM Filed June 24, 1949 ATTORNEY Patented Dec. 25, 1951 vNONLINEAR SIGNAL TRANSMISSION SYSTEM Alton J. Torre, Westmont, and Loren R. Kirkwood, Oaklyn, N. J., assignors to Radio Corporation of America., a. corporation of Delaware Application June 24, 1949, Serial No. 100,988

This invention relates to non-linear signal transmission circuits, and particularlyto means for developing biasing voltages for certain of the components of circuits of ythis character which, for example, may be employed for noise reduction purposes, so as to eifect operation of the signaltransmission circuits a predetermined region of non-linearity.

In audio signaling or signal transmission systems, it is desirable to suppress as much as possible any noise effects which may be present with the audio signals which are to be reproduced as sound effects. While it probably is true that there generally are noise effects present with substantially all audio signals, they are not particularly objectionable when the audio signals are present at relatively high levels. However. when the audio signal level is relatively low. the signal-to-noise ratio becomes appreciably large, so that the noise effects are objectionable. This is particularly true in the high frequency range of the audio signals. Consequently, When the audio signals are predominantly of high frequencies and are of low level, the noise effects become extremely objectionable.

In order to overcome, at least in part, -these objectionablecharacteristics of the noise effects, numerous noise suppression circuits have been proposed in the past.' YAmong these circuits is one covered in a copending application of Harry F. Olson, Serial No. 721,119, filed January 9, 1947, and entitled Audio Noise Reduction System. Essentially this system separates the audio signals, together with whatever noise effects may be present therewith, into tWo frequency bands. The low frequency band of signals is passed Without change from the signal source to an output circuit such as an audio signal amplifier of a sound reproducing' apparatus. The band of relatively high frequency audio signals is passed through a non-linear signal transmission circuit for the purpose of suppressing all signals Within the band having less than a predetermined amplitude. The non-linear circuit also has the characteristic of passing without substantial modication all signals within the high irequency band having greater than the predetermined amplitude. The two frequency bands of signals are then combined for impression upon the reproducing apparatus.

Essentially, the non-linear circuit consists of a pair of unilaterally conducting devices connected in parallel and in opposite polarity. In an ideal arrangement, each of the unilaterally conducting'devices would have the property of `2 Claims. (Cl. 178-#14` changing abruptly lfrom a non-conducting to a conducting state in response to voltages impressed thereon exceeding a predetermined threshold value. Thus any audio signal voltages, together with Whatever noise effects may beV present therewith,rbelow the threshold voltage of the non-linear unilaterally conducting devices will not be -reproduced in the output circuit thereof for subsequent combination With the low frequency signal voltages. Hence the noise effects are effectively suppressed at the cost, however, of concomitantly suppressing whatever signal voltages of low level may be present. Any signal voltages, together with noise effects, which exceed the threshold voltage of the non-linear circuit will -be passed therethrough Without substantial modification.

In systems of this character, it is customary to set the threshold value for the non-linear devices substantially at the maximum level of the noise effects. In such a case, there are substantially no noise effects transmitted to the 'K reproducing apparatus which would render the reproduced sound objectionable.

Considering all aspects of the performance of a non-linear circuitof the character described, it has been determined that one of the most effective elements for use as the unilaterally-conducting devices thereof is a germanium crystal. Such a device has the desirable attribute of changing at a relatively rapid rate fr-om a relatively high resistance value to a relatively low resistance value-in response to a minimum change in the voltage impressed thereon. However, the germanium crystal also has the property that, in order to perform in the manner described, it requires the impression thereon of a biasing voltage of a relatively small magnitude, such as a fraction of a volt. In systems such as that described in the copending Olson application referred to, the source of the biasing voltage for the germanium crystals is the small battery such as a dry cell or a plurality of such batteries. A biasing voltage source of such a character, however, is not entirely satisfactory for the reason that the voltagesupplied thereby ldoes not remain constant during the life of the battery, and furthermore, since such batteries have' a somewhat limited life, there is the necessity of replacing them at recurring intervals.

Accordingly, it is an object of the present invention to provide a means for developing relatively small voltages for use in biasing the unilaterally conducting devices employed in a nonlinear signal transmission circuit by deriving primarily by reasenfo theoleeiati these relatively small voltages from the relatively high voltage power supply necessary for use in conjunction with the signal amplifying apparatus with which the non-linear circuit is to be used. 5

Another object' of,` theY invention is to provide a means for deriving, from a relatively highy unidirectional voltage source, relatively small, equal voltages of opposite polarity for biasing a plurality of oppositely poled unilaterally conducting devices employed in a non-linearsignal transmission circuit.

Still another object of the inventionisto provide apparatus for deriving, Vfrom a` relatiyely high voltage unidirectional power supply, rela;A tively small voltages and for? impressing said small voltages upon the unilaterally conducting devices employed in a non-linear signal transmis-,- sion circuit in such a in anner as to render the non-.linear oireuiteindependent.-et. tiie.. inset andI s outputeireuits tnereoi.. l

Inacoordance with tnerresentinrentioni.there is. provided. a. signal.. transmission Cireuit, ei a. nonelinear Character, whereby;y to eeetemleltude. discrimination oi lternatins; voltages, lill:` pressedthereon- This ationnsi of;

at? leastlaterallr conducting devieesrouned into. sis.- nal transmission eireli't... 'Iheunilaterally e911-y dlieting devise.. property of. efieetiveli changing. its. resista. from., a relativen. hist. value to. a. relatively low. value. iii tesnense. t9. a .relatively small change in. tnearnnlitiide 0i the. voltage impressed tnereo T9. neriorrnin this manner, a relatively small,biasingvoltagewis required. to baimnressedunoneaon ofthe enilaterallycondnetins devieesfemployed.. Further, in accordance with the inveiltlo. these Diel. voltages. are. derivedirorn arelatlvely nisilvoltf ase unidirectional power supply.. through; the. agenoyofa networkoomprlslnga.relativeirhieir resistance bleeder connected across the, unidirectionalvoltage source andineludins as eetien having a.. relativelrlow eiieetiyeresistanoe-f..

In. the. easeY where.- .two..opnositelvipoledr linie laterally.` conducting devices are.: employed,... the two` terminalsof.. the relatyely 105th resistentie section, atwhichthere are developedtwd egual, relatively. low Y, voltages, of, Opposite Apolarityrela:V tive towthevoltage ofv the intelrneslaliegpptlt. Qfv the low resistance. bleederiseotiom .are oollpledte electrodes ofY opposite. polarity.l of. tlierespeetive unilaterally. Conducting devices, and a return path for unidirectional current ismconnected vbetyveen the other electrodes of these.. devices.4 andk the intermediate point ofthenlowresistange bleeder section. Additionally-, the, bleeder may; include another resistive section toisolatethegnQn-linear signal .transmission circuit t from, theIM grounded, oppositelypoled ,terminalof the, high voltage. uni: rectional power supply., A` resistorwnetworlzin accordance with. the. invention provides ayrela-- tively. simple and highly efficient ,meansgfor deriving. the necessary;biasingvoltasefor the.` laterally conducting. deviees.v whereby; to; m mlze the. current drain.plaeedunerlrthe;insti voltage, unidirectional power supply,Y It also-:1ero:l vides an arrangement which .is relatively, per: marient andthus is a marked improvement over an arrangement wherein low; voltasefbatteries at optimum effectiveness by reason of the fact i that the biasing voltages are not subject to variation as in the case of batteries o the dry cell type where the voltage decreases as the batteries age.

Further, in connection with another feature of. theV present invention, there is provided circuit; means; including a,A capacitor coupled between the signal source and the intermediate point of the low resistance bleeder section, whereby toM isolate the signal input circuit imp edance-device from the biasing voltage developing network. Also, a point on the return path for unidirectional current is bypassed to ground by meansof acapacitor so that, for the alternating. voltages, theoutput circuit of the nonlinear signal transmission circuit may be made when read in connectionA withthe accompany-v ing drawing, in which:

Figure 1` is a circuit diagram of a signal amplifying and reproducing system embodying the present invention; and

Figure 2 is a curve showing the characteristics of the. unilaterallyV conductingy device preferred for use in the illustrated embodiment of the invention.V

Having reference now to Figure 1 particularly, there is shown a signal source II which, for example, may be, any source of audio signals such asa phonographr pickup device, a signal detector of atradioy receiver, the pickup device of a film sound track,Y and thevlike. The output circuit of the signal source is4 coupledv to a low pass lter I2` which, `for,v example, may, transmit all signal voltages having frequencies from 0 to 4,000 cycles per second. The output circuit of the signal source I I also is coupled to a band pass filter I3 which, for example, may be designed to pass all signal voltages having frequencies within the range of;4,000rto 8,000 cycles per second. The output circuit of the band pass filter is coupled to a4 non-linear amplitude discriminating signal transmission circuit IIL the output circuit of which inturn isf-coupled to a second band pass filter I5, which may be similar in design to the filter I3. The` output circuits of the low pass filter I2,v and theband pass filter I5 are combined and coupled to the input circuit of a signal amplifier I6, to the` output circuit of which may be coupled a loud speaker II or other sound reproducing apparatus.v l

Theapparatus described up to this point includes in general all of the essential components of a noise suppression system of the type disclosed in the copending vOlson application referred to previously.V In effect, its operation is as follows: All signal voltages, together with whatever noise effects may be present therewith having frequencies upto 4,000 cycles per second, are transmitted without substantial modification from the signal -source I I through the low pass filter I2 to the signal amplifier I6 for ultimate reproduction by the loudspeaker I'l. All voltages including signal andnoise, effects having frequencies Within thqrangeoof 4:,000 to,y 8,000 cyclespersecond are selectedbythe bandpass filterI 3 and,transmitteri to the non-linear signal transmission circuit I4.r

This circuit includes non-linear, unilaterally conducting devices connected and biased appropriately in a manner to be described so that, for all voltages below a predetermined threshold amplitude impressed thereon, there are substantially no-signal and/or noise voltages reproduced in the output circuit for impression upon the band pass lter I 5. Consequently, only those Vsignal voltages below a frequency of 4,000 cycles per second are amplied and reproduced by the loud speaker Il. However, any signal 'and/or noise' voltages appearingin the output circuit of the band pass iilter I3` having amplitudes greaterr than the threshold amplitude are passed without substantial modification by the non-linear amplitude discriminating circuit I4 and are impressed upon the band pass lter I5. The purpose of this latter illter is to effectively remove any harmonic distortion which may be introduced by reason of the operation of the non-linear circuit III. The relatively high frequency Yvoltages developed at the output circuit of the band pass filter I5 are combined with the relatively low frequency signal voltages derivedfrom the low pass filter I2 and are impressed upon the signal amplifier for reproduction by the loud speaker I'I.

Considering now more in detail the amplitude discriminating circuit embodying the present invention, a resistor I 8 is provided as the input circuit load impedance device for the non-linear circuit I 4 and is coupled to the band pass lter I3 as shown. It will be demonstrated subsequently that the value of the resistor I8, or of any other input load impedance ydevice with which the amplitude discriminating circuit may be provided, may be'given any desired'value, by reason of the fact that the non-linear circuit and its biasing facilities in accordance with the present invention, may be entirely isolated for unidirectional currents from both the input and output circuits thereof. Essentially, the non-linear amplitude discriminating circuit in accordance with this invention comprises a pair of unilaterally conducting devices |9 and 20, both of which are connected effectively in shunt with one another and to an input terminal 2|. These devices are arranged in opposite polarity as shown, and the input terminal 2| is coupled by means of a capacitor 22 to the input load resistor I8. The non-linear circuit III also is provided with an output load resistor 23, coupled between an output terminal 24 and ground by means of a capacitor 25 having a low impedance for signal currents. The output terminal 24 of the non-linear circuit is coupled to the input circuit of the band pass filter I5.

In order to provide the necessary relatively small biasing voltages for the unilaterally conducting devices |9 and 2 I, there is provided in accordance with the present invention a network for deriving these voltages froml a unidirectional power supply 26 of suiiiciently high voltage to furnish space current for the signal amplifier I 6 and other apparatus of a similar character. As conventionally employed, the negative terminal of the power supply is grounded. The positive terminal of the power supply is connected to one terminal of a relatively high resistance bleeder 2'I, the other terminal of which is grounded. The bleeder 21 comprises the series connection of a plurality of resistors including a resistor 28 having a relatively high value, two equal resistors 29 and 30 forming a relatively low resistance section, and a resistor 3| of substantially any desired value. Preferably, the value of resistor 3| should be approximately ten times the value of 6, the input load resistor 8. The input terminal 2| of the non-linear circuit I4 is connected to the junction point between the relatively low, equal valued resistors 29 and 30, which point also is connected by a resistor 32 to the junction point between the output load resistor 23 and the capacitor 25.

Before considering in any greater detail the non-linear amplitude discriminating circuit I4, it is considered that a better understanding of the present invention may be had by rst considering the essential properties of the unilaterally conducting devices I9 and 20. It has been determined that, for employment in a circuit of the present character, germanium crystals provide themost eiective devices for accomplishing the desired results. By reference to Figure 2, it may be seen that germanium crystals are capable of operating in a very eiiicient manner. This iigure illustrates the non-linear conducting property of a germanium crystal. The curve 33 illustrates the change in resistance'of such a crystal in response to changes in the voltage applied thereto. In this gure, the applied voltages are plotted as abscissas and the resistances are plotted as ordinates. It will be noted that, when no voltage is impressed upon the crystal, it has a resistance value of approximately 10,000 ohms. Also it will be observed that, if a relatively small negative biasing voltage of, say, approximately 0.3 volt is impressed upon the crystal, it has a resistance of approximately 100,000 ohms, as indicated by the point 34. Also, in response to a relatively small voltage of positive polarity, the resistance of the crystal becomes quite small, such as of the order of ohms. Therefore, if such a crystal be initially biased by the impression thereon of a negative voltage having a magnitude of something less than l volt, and the impressed voltage varied to a relatively small degree, say, to any voltage of positive polarity, the resistance of the crystal will change from a value of the order of 100,000 ohms to one of the order of 100 ohms. For all practical purposes then, in response to voltages having magnitudes less than a predetermined one, th-e crystal is virtually nonconducting and, in response to any voltage having a greater' than the predetermined magnitude, the crystal is highly conducting. Consequently, it is well suited for use in an amplitude discriminating circuit of the character disclosed herein. However, in order to be of optimum utility, it is necessary to bias such a crystal by means of a relatively small magnitude voltage of negative f polarity.

The derivation of such unidirectional biasing voltages of proper polarity, for impression upon the unilaterally conducting devices I9 and 29 of Figure l, is one of the functions of the bleeder 2l. In developing such voltages, it also is desirable to minimize the current drain upon the power supplyA 26. Consequently, to effect both of these results, the bleeder must of necessity have an overall resistance which is relatively high to minimize current drain upon the power supply and yet, at thesame time, must includ-e a relatively low resistance section in orderto develop the relatively small voltages required for biasing purposes. The first requirement of such a bleeder is met principally by means of the relatively high valued resistor 28. The second requirement is met by the connection in series with the resistor 28 of the relatively low resistors 29 and 30. One of the desirable attributes of germanium crystals for use inan amplitude discriminating circuit of the character employedir the present'system is that they may be procured in substantially perfectly matched pairs. As a consequence', the characteristics of each of' the devices' i9 and 25 are substantially identical, so that equal ixed resistors .25 and 3Q may be employed' to develop substantially equal biasing voltages for the` unilaterally conducting devices;

In order to suitably bias each of the unilater' ally conducting devices I9 and 2li so that, in the absence of voltages of greater than' a predetermined magnitude, they will exhibit a relatively high resistance, itis necessary that the cathodes thereof be of positive polarity' with respect'to' the associated anodes. Specically, in the device i9 the cathode -ic should be of; positive polarity with respect to the associated' anode' c'. Similarly, the cathode it" of the device 2i! .should be of positive polarityV with respect to its associated anode Considering that the current' now from the positive terminal of the power supply Eli through the bleede'r 2l is in the direction of the arrow, it will be seen that the upper terminal of the resistor 29 is of positive polarity with respect to the lower terminal thereof. This latter terminal is conductively connected by means of resistors 23 and 32, serving as a common return path for both oi the unilaterally conducting devices, to the anode a, of' the device i9, while the cathode l: thereof is connected to the upper terminal of the resistor 29. :i

Therefore, in view of the' relative polariti-es or' the two terminals of the resistor 2?, itis seen that the device i9 is properly biased'. Also the current now through the bleeder 2'! traverses resistor 3u in such a direction to make the upper terminal thereof positive with respect to the lower termi nal. The lower terminal is connected, as shown, to the anode a of the device 2G and the upper terminal or the resistor @El is connected through the common return path including resistors 23 and 32 to the cathode 7c. Here again it is seen that, in view of the relative polarities of the terminals of resistor S0, the device 2t is suitably biased to exhibit a relatively high resistance.

Therefore, when the signal and/or noise'` volt'- ages developed across the input load resistor I8, for the band of frequencies passed by the filter' I3, are less in magnitude than the biasing voltages impressed upon the unilaterally conducting devices I9 and 20 in the manner described, the c amplitude' discriminating circuit I4 is virtually non-conducting, so that none of the alternating voltages appearing in the output circuit of the band pass lter I3 are transmitted to the input circuit of the band pass nlter I5. Consequently, there is no reproduction of these voltages by the amplifier I6 and loud speaker il. However, when the alternating current voltages developed in the resistor' I8 exceed in magnitudeV the biasing voltages oi the unilaterally conducting devices IS and 20, the circuit I4 substantially instantaneously becomes highly conducting, so that there is substantially unmodied reproduction of these voltagesv by the amplier I6v and loud speaker I1.

Inasmuch as the non-linear amplitude discriminating circuit I4 functions essentially as a clipping device, there are necessarily produced, as a result thereof, distortions of the alternating current voltages appearing in the input load resistor 23. These distortions' are produced by reason of the development of harmonics of the fundamental voltages impressed upon thel input circuit load resistor I8.y By reason of the connection of the unilaterally conducting devices I9 and /20in opposed signal ampler I6, the additional bandl pass iil-v ter I5; This lter serves to effectively eliminate' substantially all odd harmonic distortion.

It may be seen, therefore, that the present invention provides an eiiiciently operatingV nonlinear amplitude discriminating circuit for use in' effectively suppressing' all voltages impressed thereonhaving a magnitude which is lessthan a predetermined magnitude,- and at the same time serves to transmit'to thej reproducing apparatus.l substantially' unmodified, all impressed voltages having' magnitudes' greater than the predetermined one'. The amplitude discriminating circuit'.

by including devices' such as germanium crystals as the non-linear unilaterally conducting devices, preferably should be provided with suitable facilities for impressing biasing voltages of relatively small magnitude upon the crystals. The facilities, in accordance ,with the presentinvention, for providing these relatively small biasing voltages are of such a character that they may be derived from a relatively high voltage unidirectional power supply. Furthermore, the voltage developing facilities in accordance with this invention are of such a character that the unidirectional currents thereof may be electively isolatedfrom both the input and output cir cuits of the non-linear. circuit. Therefore, the

biasing facilities function entirely independently' of the input and output circuits of the amplitude discriminating circuit. Such a circuit,v consequently, may be employed with input and output circuits of any desired character withoutaf fecting the input and output circuits or thefacilities for developing. the biasing voltages. Furthermore, the biasing voltage developing facilities, in accordance with this invention,.are high.-r ly efcient in their operation and, therefore,` do not place any substantial additional drain upon the power supply. Y

Without in any way limiting or restricting .the present invention, the following table of valuesv of the essential components of therillustrated ernbodiment of theinvention is given by way of eX- ample.:

Resistor I8 500 ohms Crystals I9, 2li Sylvania- 1/N35 Capacitors 22, 25 Y 0.5 microfaradV Resistor 23 10,000 ohms- Power supply 26 250 volts Resistor 28 820,000 ohmsv Resistors 29, 30' 1,000 ohms Resistor 3 I 4,700 ohms Resistor 32 100,000 ohmsY From this table it may be seen .that rthe additional drain placed upon .the power supply 2li by the inclusion of the disclosed network for developing the relatively small biasing voltages for the unilaterally conducting devices I9 and 20. is comparatively small, and is ofthe order vof'O milliampere. Also it may he seen that this current, in traversing the relatively low valuedfresisters 29 and St, develops a vbiasing voltage of approXimately 0.3 volt. Furthermore, it will-foeobserved that thesignal voltages are impressed upon a circuit including effectively theseries connection of one of theequa'l resistors 2,9 or' 30 .the (561"resp()ndlr/1g4 unilaterally" Conducting device fg' or 20, and the output load resistor 23. In view of the fact that, when the devices I9 and 2Q are in a conducting state, the resistance thereof is negligible, it indicates that the signal voltages are impressed upon a circuit including essentially the resistors 29 or 39 and the resistor 23. Since this latter resistor, in accordance with the foregoing table, is vapproximately ten times the value of either of the resistors 29 and 30, it may be seen that at least 90 per cent of the signal voltages are developed across the output resistor 23. Accordingly, the amplitude discriminating circuit, including the resistive biasing network therefor, operates in a highly eiicient manner.

It will be appreciated that the present invention may be embodied in forms other than that specifically described herein for illustrative purposes. It is to be understood, therefore, that the presently disclosed embodiment of the invention is not to be considered necessarily as limiting the scope of the invention. Accordingly, the scope of the invention is to be determined by reference to the appended claims.

What is claimed is:

1. In a signaling system including a source of alternating voltages of variable amplitude, a band pass illter circuit having an input circuit connected to said source and an output circuit, an amplitude discriminating circuit comprising a pair of shunt-connected, oppositely poled nonlinear unilaterally conducting devices, each having the property of being substantially non-conducting when energized by a voltage of predetermined polarity and relatively small magnitude and highly conducting when energized by voltages of opposite polarity, a source of unidirectional voltage of relatively large magnitude, a network for deriving from said unidirectional voltage source a voltage of said predetermined polarity and relatively small magnitude, said net- Work comprising a relatively high resistance bleeder connected across said unidirectional voltage source, said bleeder including a relatively low resistance section comprising two substantially equal serially connected resistors, connected to said unilaterally conducting device for impressing said relatively small voltages upon said devices, an input load impedance device connected to said output circuit, a capacitor connected between said input load impedance device and the junction of said low resistance resistors and isolating said impedance device from unidirectional currents of said network, and a return path for unidirectional network current including an output load rnpedance device of a magnitude several times said input impedance, said output load impedance including a pair of serially connected resistors connected between the junction of said low resistance resistors and the junction of said pair of unilateral conducting devices, and a capacitor connected between the junction of said output load impedance resistors and one termina1 of said source of unidirectional voltage, and

1G a band pass lter circuit having a characteristic substantially identical to said rst band pass lter circuit and having an input circuit connected to said unilateral conducting device junction and said one terminal of said source of unidirectional voltage and an output circuit.

2. In a signaling system including a source of audio signal voltages of variable amplitude, an input load resistor coupled to said source, an amplitude discriminating circuit comprising a pair of shunt-connected, oppositely poled nonlinear unilaterally conducting germanium crystals, each having the property of being substantially non-conducting when energized by a negative voltage cf relatively small magnitude and highly conducting when energized by negative voltages of smaller magnitudes or by positive voltages, circuit means including a capacitor connecting said input load resistor to said amplitude discriminating system, a source of unidirectional voltage of relatively large magnitude, a network for deriving from said unidirectional voltage source a negative voltage of said relatively small magnitude, said network including a relatively high resistance bleeder connected across said unidirectional source, said bleeder including a relatively low resistance section comprising two serially connected equal resistors, said bleeder also including two resistors having relatively high values of resistance connecting the terminals of said low resistance section to the terminals of said unidirectional voltage source, substantially non-resistive circuit means connecting the terminals of said low resistance section to oppositely poled electrodes of said respective devices, a return path for unidirectional current including an additional resistor of relatively high value connected between the other electrodes of said devices and the junction point between said two equal resistors, an output load resistor also included in said return path and having a relatively high resistance as compared to that of said input load resistor, and a capacitor connecting said output load resistor to ground for audio signal voltages.

ALTON J. TORRE. LOREN R. KIRKWOOD.

REFERENCES CITED The following references are of record in the 'lle of this patent:

UNITED STATES PATENTS Number Name Date 2,171,671 Percival Sept. 5, 1939 2,342,238 Barney Feb. 22, 1944 2,423,263 Sprague July 1, 1947 OTHER REFERENCES Article: Audio Noise Reduction Circuits, by H. F. Olson, published in December 1947 Electronics, pp. 118-122. 

